Synchronizing system



sepas, 1946.

K. R. WENDT Filed'July 29, 1944 I SYNCHRONI-ZING SYSTEM 5 Sheets-Sheet 2 PE2/00.02 55C.

aurpurfeonu'm Le)l i @Waffen/160%# /z/M/rfo 5 Sheets-Sheet 3 K. R. WENDT sYNCHRoNIzING SYSTEM Filed July 29, 1944 Sept. 3, 1946.

.fyvcr Nirwo Filed July 29, 1944 5 Sheets-Sheet 4 INVEN TOR. )r-RWM BY 2M HTTIPA/EY Sept. 3, 1946.

I K. R. WEN DT SYNCHRONIZING; SYSTEM Filed July 29, 1944 nnnllnnflfnn 5 Sheets-Sheet 5 /f/fwmm IN V EN TOR.

A TKA/E Y Patented Sept. 3, 1946 SYNCHRONIZING SYSTEM Karl R. Wendt, Hightstown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 29, 1944, Serial N0. 547,225

12 Claims.

The present invention relates to secret telecommunication systems and more particularly to an improved method of and means for generating and synchronizing complex signals which may be utilized, for example, as coding and decoding waves in such systems.

The invention, by way of example, will be described hereinafter as an improvement in the synchronizing of coding wave generators of the dual delay network type which may be employed in a secret telecommunication system of the type described in the copending U. S. application, of Alda V. Bedford, Ser. No. 546,189, filed July 22, 1944. Said copending application discloses a system wherein, for example, a speechsignal comprising a complex wave S is modied by means of a coding signal comprising a complex wave K in a manner whereby the instantaneous ordihates of the resulting coded signals are the product SK of the corresponding instantaneous ordinates of the speech signal and the coding signal with respect to their A.C. axes. The coding Wave K is generated by pulse-exciting separate delay networks at diierent rates, selecting predetermined delayed pulse components from each network, multiplying the component waves together, and distorting the resultant product wave. The resulting unintelligible coded signals are transmitted by any conventional means to a receiver wherein the coded signals are combined with decoding signals generated in the receiver and having instantaneous ordinates corresponding to the reciprocal of the corresponding instantaneous ordinates of the coding signal component of the transmitted signal. The decoded signals are derived from the product of the transmitted signal SK and the decoding signal l/K. Each of the component pulse-excited delay networks of the coding and decoding signal generators at the transmitter and receiver, respectively, are disclosed in said copending application as synchronized separately by means of special synchronizing pulse signals each comprising a first signal pulse immediately followed by a second signal pulse of opposite polarity, which pulses for each component network occur at diierent rates and may be superimposed upon the coded signals SK. At the receiver, the reversals in polarity between the two synchronizing pulses are employed to synchronize separately each of the component delay networks of the decoding wave generator.

rlhe instant invention comprises an improvement over the coding wave generator described in said copending application, in that synchronizing pulses for only one of the delay networks need be transmitted with the coded message signal. At both transmitter and receiver, the remotely synchronized networks each provide local auxiliary synchronizing signals which are employed to lock-in the other local pulse-excited network at the predetermined diiierent pulse rate. The local auxiliary synchronization is accomplished by deriving a harmonic wave in a novel manner from each of the remotely synchronized networks, and by employing each of said harmonic waves locally to lock-in the other local network at some predetermined sub-harmonic rate.

In order to change the coding wave continuously, the delayed pulses at a plurality of points in each of said networks are combined in predetermined polarities by a continuously-changing, diierential-speed, selecting mechanism to provide a complex wave having a network periodicity of about .4 second which, by means of the switching, may be extended to over one hour.

Among the objects of the invention are to provide an improved method of and means for synchronizing a plurality of wave generators. Another object of the invention is to provide an improved method of and means for synchronizing a plurality of different frequency generators at a plurality of separated locations by means of a single synchronizing signal. Another object of the invention is to provide an improved method of and means for synchronizing secret telecommunication systems. An additional object is to provide an improved method of and means for synchronizing a plurality of different frequency generators by means of a single transmitted synchronizing signal and a plurality of locally generated synchronizing signals for synchronizing said generators at each of a plurality of separated locations. `A further object of the invention is to provide an improved receiving network for synchronizing one of a plurality of diierent frequency wave generators, the remainder of said generators being synchronized by local synchronizing signals controlled by a received synchronizing signal. A further object of the invention is to provide an improved method of and means for synchronizing a plurality of wave generators including a delay network excited by waves from one of said generators, wherein a harmonic signal is produced from successively delayed wave components derived from said network, and wherein another of said wave generators is synchronized at a sub-harmonic frequency of said harmonic wave.

The invention will be described in greater detail by reference to the accompanying drawings of which Figure 1 is a schematic, partly block diagram of a complete secret telecommunication system employing the invention,

Figure 2 is a series of graphs illustrating the operation of the transmitter synchronizing circuits of one of the wave generators of the system of Figure 1,

Figure 3 is a series of graphs illustrating the operation of one of the decoding wave generators of the receiver portion of the system illustrated in Figure 1,

Figure 4 is a schematic, partly block circuit diagram of a component code wave combining circuit comprising aV portion of the circuit of Figure 1,

Figure 5 is a series of graphs illustrating the operation of the combining circuit illustrated in Figure 4,

Figure 6 is a graph illustrating the local synchroni'z/ation signal derived from one delay network for synchronizing the second delay network at each local station,

Figure 'l isa schematic circuit diagram of the multiplier circuit forming portions of the circuits shown in Figures l and 4, f

Figure 8 is a schematic circuit diagram of the reciprocal circuit forming a portion of the circuit shown in Figure 1,

Figure 9 is a side elevational view of one of the rotatable contacto-r disks forming a portion of the code wave switching mechanism, and

Figure l0 is a side elevational view of one of the ijxed brush holders forming another portion of said switching mechanism which operates complementarily with said contactor disk.

Similar reference characters are applied to sim-l ilar elements throughout the drawings.

CODING WAVE GENERAFTOR Generator unit-A Referring to Figure 1, the coding wave generator which may be employed alternately for both transmitting and receiving coded speech signals includes a conventional, free-running multi-vibrator circuit I which generates pulses at a rate', for example, of 50 pulses per second. A typical multivibrator of this type, the frequency ofwhich maybe controlled by recurrent applied control pulses, is described in U. S. Patent 2,266,526, granted to E. L. C. White on December 16, 1941. The generated pulses are applied to the inputof a rst conventional delay network 2- comprising' a plurality of series-connected inductors 3, 5, "I, 9 II and a plurality of shunt-connected capacitors 4, 6, 8, Ii), I2, M. The remote terminals ofthe resultant pulse delay network 2 are terminated by a resistor I3 matching the surge impedance of the network. It should be understood that the delay network 2 may include, for example, sixty lter sections 'as indicated by the dash lines interconnecting the filter sections Il, I 4 and the terminating resistor I3, and that equalizers and booster amplifiers may be inserted in the delay network at desired points to maintain pulse amplitude relations at optimum values.

Pulses applied by the multivibrator Iv to the input of the delay network'2 provide similar pulses at the junction of eachof the succeeding series inductors 3, '1, 9, II, etc., whereby each succeeding pulse is delayed a predetermined amount with respect to pulses occurring at'other prior network terminals. A.complex-'coding wave thus may be obtained in response to each pulse applied to the delay network by combining in either or both polarities differently delayed pulses derived from a plurality of such predetermined points along the delay network.

Separate isolating resistors I5, I1, I9, 2|, 23, 25, each have one terminal connected to different points along the delay network, and have their remaining terminals connected to separate movable contacts of a plurality of single-pole double-throw switches 2l', 29, 3l, 33, 35, 3l. The corresponding fixed contacts of the several switches are connected together to provide two lines 39, 4|, which are terminated through resistors 43, l5 respectively to ground. The remaining terminal of the line S9 is connected through a coupling resistor d1 to the input of a code Wave combining network 49 which may be of the type described in detail hereinafter by reference to Fig. 4 wherein the first component of the coding wave is combined with a second component of the coding wave which second component is derived from a second coding Wave generator B of the same general type as said rst coding Wave generator A described heretofore but which is excited at a diierent frequency, such, for exam ple, as 45.4 cycles.

The output of the code wave combining network is connected to one ixed contact T1 of a iirst transmit-receive single-pole, double-throw switch 5d. The remaining terminal of the second line 4I is connected through a polarity reversing amplifier 5I and through a second coupling resister 5.3 to said rst coupling resistor il and tov the input of the combining network 49. Thus each of the pulses derived from the multivibrator I and applied to the input of the delay network 2, provides a plurality of pulses of either polarity occurring at predetermined intervals during each applied pulse period, as determined by the points of connection to the delay network and the arrangement of the switches 2l, 29, 3l, 33, 35, 3l. Therefore, a complex coding wave component may be applied to the input circuit of the code wave combining network 49 merely by selecting the desired arrangement of the pulse selecting switches. It should be understood that the total delay provided by the pulse delay network should be at least slightly less than the normal pulse period of the Ymultivibrator I in order that only onepulse may be traveling along the delay network at any predetermined instant.

Thus far, the coding Wave generator A is similar to that described in the copending application identined heretofore.

CoDINc TRANSMITTER Referring to Figures 1 and 2, the system may be employed as a coding transmitter by switching the. movable contacts of each of the single-pole,

Vwill be described indetail hereinafter by reference to Figure 7 of` theY drawings. Coding signals, comprising component waves provided by the coding wave generators A and B are derived from the Vcoding sig-nal generator combining network 49, and are applied, through the nrst switch 5G, to a secondv input circuit of saidwave multiplier 59,wh`ereby coded signals SK having instantaneusordinates corresponding to the products of the corresponding instantaneous ordinates of the speech signal S and the coding signal K are applied through a third transmit-receive switch El `to`one input circuit of a rst signal mixer circuit 63, which may comprise any conventional network wherein applied signals are combined algebraically. i

TRANSMITTER SYNcHRoNIzINc PULSE GENERATOR synchronizing unit A Regularlyv recurrent pulses indicated by the graph a ofjFigure 2 are derived, for example, from thegxfifty-ninth tap on Ythe delay network 2V of the'code wave generator A and applied to a conventional ,thermionic tube limiter circuit` 61, which' clips the wave a at the level w to derive individual limited pulses represented by graph b of Figure 2. The limited pulses b are applied through a fourth transmit-receive switch 69 to keya second multivibrator 1| to derive a negative,. substantially square-wave pulse illustrated by graph c of Figure 2. The negative square wave pulse cis applied through a fth transmitreceive switch 13 to a second input circuit of the first signal mixer circuit d3, and is applied through a sixth transmit-receive switch to key a third multivibrator 'l1 which generatesa positive square wave pulse indicated by the graph d of Figure 2. It will be understood that the positive square wave pulse d will be initiated at the termination of the negative square wave pulse c in a manner well known in the multivibrator art. The positive square wave pulse d is applied to a third input circuit of the signal mixer circuit 63 whereby the coded signal SK, the negative square wave pulse c and the positive square wave pulse d are combined to provide a communication signal including the coded wave and the main synchronizing signals. The combined coded signal and synchronizing signals derived from the signal mixer 83 will have a waveform, for example, of the type illustrated in graph f of Figure 2, including the pulses I, I, shown in dash lines. The coded signal SK and synchronizing pulses I, I are clipped at levels 1I, II in a limiter 19 and are applied through a conventional radio transmitter 8l to an antenna 83. As received the Waveform may resemble the wave 1:.

A pulse derived from the third multivibrator 11 also is applied to key the nrst multivibrator l to generate a positive square wave pulse e, illustrated in Figure 2, which is applied to the input of the delay network 2 to initiate a succeeding pulse which will be progressively delayed along the delay network. Since the iirst multivibrator I is keyed by the pulse from the third multivibrator 11 immediately preceding the time for the generation of a normal pulse by said rst multivibrator, it will be seen that the coding wave generator will be self-running, and will be maintained at a substantially constant frequency since the pulse rate therethrough will be substantially dependent upon the time delay of the successive pulses applied to the delay network 2. If the first multivibrator l is not properly keyed by the third multivibrator Tl, the first multivibrator merely will generate a pulse e which will be applied to the delay network 2 at a slightly later interval. The slightly delayed pulse upon reaching the fifty-ninth tap of the delay network therefore will key the second and third multivibrators in the manner described heretofore, and will provide a new set of synchronizing pulses` Cil 6 which will actuatethe first multivibrator l in synchronism thereafter.

ConING WAVE GENERATOR B The second coding wave generator B comprises a second delay network 2 -excited by pulses from a multivibrator l' which has a normal operating frequency, for example, of 45.4 cycles. `.The delay network 2 is similar in all respects to the first delay network 2, with the exception that its totalV delay is of the order ofv 1&4. second. The second code wave generator B' includes isolating resistors, network switching elements, polarity reversing amplier, and terminating resistors, not shown, of the same types Vand connected in the same manner as describedA heretofore with respect to the rst code wave generator AL Signals derived from the network switches and from the polarity reversing amplifier of then generator B', (which may be connected `asshown in the first code wave gen- SYNCHRONIZATION oF CODING WAVE GENERATOR B' A harmonic frequency signal, such, for example, as 500 cycles,'may be derived from the 50 cycle-excited rst delay network 2 by deriving signal components from successive network sections and by reversing the polarity of predetermined'ones of said derived delayed signal components. For example, resistors I5, Il" and I9" connected to successive delay points on the delay network 2, if properly proportioned, will provide a half wavelength signal having a period of V the delay period of the entire 60 section network 2. Similarly, resistors 2i", 23" and connected to the next three successive points on the delay network, may be connected through a thermionic tube signal inverter 2d to provide a second half wavelength signal of opposite polarity but having the same periodicity as the rst half wavelength signal derived from the resistors l5", Il" and I9. If the signals derived from the first group of resistors are combined with the inverted signals derived from the second group of resistors a complete cycle of a wave is produced which is the tenth harmonic of the cycle exciting frequency applied to the delay network Successive delay points on the delay network 2 therefore may be connected through additional suitable isolating resistive groups, and combined in proper `polarity to provide successive positive and negative half cycles of the tenth harmonic wave, in order to provide a substantially continuous harmonic signal, as shown in the graph of Fig. 6, which may be employed for synchronizing the second code wave generator B'. If desired, selected spaced delayed pulse components of a single polarity may be derived to provide a harmonicwave comprising only half cycles oi one polarity.

The graph of Fig. 6 shows the combination of differently delayed, differently polarized and different amplitude exciting pulses which approximate a harmonic sine wave. Actually, each exciting pulse 'acquires a tail upon being delayed, whereby the summation of the exciting pulses and their tails in selected delayed relation closely 7 approximates the sine wave illustrated. Filtering may further improve the waveform'.

The thus derived tenth harmonic 500` cycle local synchronizing signal may be applied to excite the multivibrator l' of the second code wave generator '.B at its eleventh sub-harmonic toprovide a 45.4 cycle signal which may excite the second delay network 2 in step with the 50 cycle excitation of the first delay network 2. `Thus the excitation of the two delay networks may be phased by means of the generator harmonic local synchronizing signal, and the single group of synchronizing pulses described heretofore may be transmitted with the coded signal for synchronizing the second wave generators of a remote receiver.

l CoDED SIGNAL RECEIVER In order to convert the circuit thus described to operate as a decoding signal receiver, the/mov-v able contacts of each of the transmit-receive switches 5e, 5l, 6I, 69, 'l3, and 15 are switched to the Vcorresponding fixed contacts Di, D2, D3, D4, D5, De corresponding to the receive condition. The combined coded signal and principal synchronizing signals transmitted from the trans-- mitter 8l are smeared and phase-shafted somewhat in transmission to resemble the solid portion a: of the graph f of Fig. 2, and as received -by means of a conventional radio receiver 85 are applied to a conventional Wave differentiating network 8l which may be of any type well knownA in the art. ForV example, a wave may be differentiated by applying it to a network comprising a small series capacitor and shunt resistor. The received signal :l: of Figure 2 after being differentiated at the receiver resembles the graph g of Figure 3 wherein a relatively large pulse P occurs at an instant corresponding to the reversal in polarity between the received synchronizing negative and positive pulses and wherein low frequency components are substantially removed from the pulse P. It should be understood that instead of differentiating the received signal, it may be treated in any other known manner to derive a pulse in response to the reversal in polarity of the negative and positive synchronizing pulses.

The iirst multivibrator I, being free-running during transmission and reception,l will apply pulses to the delay network and the delay network 2 will provide recurrent pulses at its nity-eighth tap which will be limited by means of a third limiter 89 to provide limited pulses represented by the graph h of Figure 3. The thus limited pulses h are applied to key a fourth multivibrator 9i which generates a relatively long blanking pulse illustrated in graph z' of Fig. 3. VThe long blanking pulse i is applied to a blanking circuit 93 which blanks out the synchronizing pulsev portions of the received signal as will be explained in greater detail. hereinafter.

RECEIVER SYNCHRONIZING' CIRCUITS Unir A Similarly, each of the recurrent pulsesv derived from the sixtieth tap of the delay network 2 are applied to a fourth limiter 95 which clips the upper portion of vthe applied pulse as explained heretofore with respect to pulse b, to provide a similar, but later, short pulse illustratedr by graph fi of Fig'. 3; The limited pulse 7' is applied throughV the fourth transmit-receive switch GS'to key the second multivibrator 1I tio provide a rela.-

,the second. multivibrator 1I is applied through the fifth transmit-receive switch l'3` to a second mixer circuit 91, to which also is applied the differentiatedV wave g derived fromv the differentiating circuit 8l.r The thus mixed signals illustrated by graph l` of Figure 3 include a pulse peakl which corresponds in time to the occurrence of" the largepositive pulse P of the differentiated received vwave y'. As explained heretofore, the pulse P Ycorresponds to the reversal in polarity of the received synchronizing negative and positivepulses. The wave Z derived from the.Y second mixer circuit 91 is applied to a fifth limiter 99' which clips the mixed signal at a level y to provide in its output circuit a short somewhat triangular pulse, illustrated by graph m of Figure 3.

The triangular pulse m is applied through the sixth transmit-receivev switch 15 toV key the third multivibrator TI to provide a positive pulse, represented by graph n of Figure 3, which is applied to key the first multivibrator l aS described. heretofore with respect to the pulse d in the transmitting network. It should be understood that, if desired, for extremely precise synchronism, the pulse m may be changed from triangular to square wave shape by clipping at a lowlevel, and then by amplifying the clipped lower portion of the pulse in a manner known in the art. The pulse n therefore causes the rst multivibrator l to generate a positive pulse o which is applied to .the delay network 2 in the same manner as described heretofore with respect to the positive pulse e of the transmitting network.

As explained heretofore with respect to the operation of the multivibrator circuits in the transmitting condition, if the circuit falls out of synchronism, the various multivibrators will provide pulses at somewhat increased time intervals until such time as a synchronizing pulse occurs at a proper instant to pull all of the multivibrators. back into synch-ronism. Since pulsesV are derived from .the delay network 2 at intervals of .the order off .02 second, it is apparent that the Various circuits will fall into synchronism in a relatively short time which seldom will exceed one full second.

Due to phase distortion in the transmission or radio circuit interconnecting the transmitter and receiver units, it is possible that the effective time of occurrence of the received synchronizing pulses will vary in different receivers with respect to the received coded speech. To correct for such variations, the circuit constants of the third multivibrator 'l1 may, in any known manner, be altered in the receiving condition so that the width of the pulse n may be varied to pr-ovide keying of the rst multivibrator I at the precise desired instant. The manner of varying the circuit constants of multivibrators to provide pulses of desiredV polarity and duration in response to predetermined applied keying pulses isknown inthe art.

" RECEIVER CODING WAVE GENERATOR B I'he second coding wave generator B of the receiver comprises the identical elements employed for the purpose in the coding wave transmitter described heretofore. No switching is necessary to convert the second code wave generator B' from the transmitting to the re- B may be synchronized with the code wave gen- Y erators at the transmitter by means of the-single group` of synchronizing pulses transmitted with the coded signal. As in the transmitter, the output of the second delay network 2' of the second code wave generator B' is applied to the code wave combining network 49 to provide the oomplex code wave K which may be changed to reciprocal form as described herein for decoding the received coded signal.

It should be understood that the harmonic local synchronizing signal may comprise harmonic pulses all of the same polarity, in which case the inverting amplifier 24 may be omitted at both transmitter and receiver. The wave shape of the harmonic synchronizing signal may be varied by limiting, or by ltering, or by varying the connection points on the first network 2, as desired. Similarly, the harmonic relation may be varied by proper selection of delay points on said rst network 2.

SIGNAL DEcoDINo SYSTEM The received signals derived from the radio receiver 85 are applied to the input of the blanking circuit 93 which interrupts the received coded signals during the time intervals of the recurrent blanking pulses i, whereby the transmitted positive and negative synchronizing pulses for the code wave generator A may be removed from the received coded signal. This condition obtains when the coding signal generators of the receiver are in synchronism with the transmitter coding signal generators, since the fourth multivibrator 9| is responsive to pulses derived from the second from last tap on the first delay network 2.

Blanking circuits are well known in the art. They may comprise, for example, a push-pull amplifier for the signal, arranged so that the blanking pulses i are superimposed on the gridcathode circuits so that both tubes are simultaneously driven to cut-oir during the blanking period.

The thus-blanked, received signals comprise the transmitted signal components SK which are applied through the second transmit-receive switch 51 to one of the input circuits of the wave multiplier 59..

Similarly, the combined coding signals K generated by the receiver coding generators and code wave combining circuit 49 are applied to the input circuit of a reciprocal circuit IUI, which will be described in detail hereinafter by reference to Figure 8 of the drawings. Signals derived from the reciprocal circuit IBI will have instantaneous ordinates corresponding to the reciprocal values of the instantaneous ordinates of the synchronized coding wave K generated in the receiver. The reciprocal wave l/K is applied through the 10 first transmit-receive switch 50 to a second input circuit of the multiplier 59. Since the wave multiplier 59 provides output signals which have instantaneous ordinates corresponding to the product of the instantaneous ordinates of the waves I/K and SK applied thereto, the output signals applied through the third transmitreceive switch 5I to a reproducer H23 will be substantially characteristic of the original speech modulation signals S (not graphically illustrated). The signals applied to the reproducer |03 have been characterized as S' since some phase distortion is inherent in the various circuits described, and especially is encountered in many radio transmission circuits. It should be under- Vstood that the signals S derived from the third transmit-receive switch 6I may be applied to actuate any other desired type of utilization apparatus, not shown.

In Figure 4, a preferred type of component code wave combining circuit comprises two separate code wave generators A' and B', having separate multivibrators I, I which excite them at diierent frequencies, such, for example, as 50 pulses per second and 45.4 `pulses per second, which are, respectively, the 10th and 11th submultiples of the frequency of the 500 cycle local synchronizing harmonic wave sh'own in Fig. 6. The operation of the individual component code wave generators, and the synchronizing thereby by both transmitted and local synchronizing signals has been described heretofore. V l

The first ycode wave generator A' may include, for example, sixty delay network sections of the type, described heretofore in connection with Figure 1, having a total delay of 1/50 Second which results in a cutoi frequency of the order of 660 cycles. The second code wave generator B' may include, for example, fifty-eight delay network sections lh'aving a similar cutoff frequency, but providing a. delay of the order of 1/45.4 second.

As a matter of convenience, circuit components of the second code wave generator B are given primed reference characters corresponding to the f component reference characters in the first code wave generator A'. The coding switches 2l, 29, 3l, 33, 35y 31, and 21', 29', 3|', 33', 35', 31', for selecting delayed pulses from preselected terminals of both delay networks, may be combined in a differential-speed, multi-section switching unit of the type described in said copending application and partially shown in Figures 9 and l0 which are described in detail hereinafter.

For each position of the coding switches of the delay network 2, an irregular wave is generated at both' the sending and transmitting apparatus. This wave is transmitted over the lines 39 and 4I to an amplifier-limiter circuit |09 in a manner whereby the differently delayed components of the wave are applied in either polarity to the amplifier input circuit. This may be accome plished in any manner known in the art, such, for example, as shown in the circuit of Fig. 1, or by coupling the lines 39 and 4I to the grid circuit and cathode circuits, respectively, of the ampliiier circuit. The sharp limiting action of the amplifier-limiter |99 produces a corresponding irregular rectangular wave K1 shown in graph F of Figure 5. In a similar manner the second delay network 2 is connected to a. second ampli- Iier-limiter III through the lines 39', 4I' whereby a second irregular rectangular wave K2, shown in graph G of Figure 5 is generated.

The rectangular irregular waves K1 and K2 are applied to a, multiplier circuit II3, of the'type shown and described herein with respect to Figure 7 of the drawings, to derive a more complex rectangular wave K12 shown in graph I-I of Fig- .ure 5.

Th'e complex wave .K12 will have a period of .4 second although it is composed of two waves K1 Yand K2 each having periods of the order -of .02 second. Also, the complex wave .K12 will have the combined number of crossovers .of the A.-C. axis of the rectangular waves K1 and K2. fSince, however, the Crossovers of the `complex wave K12 have short term repetition rates of the order of 1/4s.4 second and 1/50 second, it is necessary to .distort the wave further to insure transmission security.

. .lo 'Ihe limiting action of the limiters rH39 and Hl vices which provide an instantaneous output voltage which is proportional to the square lof th'e instantaneous input Yvolta-ge over a reasonable voltage range in a single polarity. Such vcircuits, or devices will be referred to as Squaring circuits, and will be designated as Q1, Q2, Q3, Q4, where referred to hereinafter.

In the preferred form of the multiplying circuit, the waves S and K, for example, to be multiplied, are added together with four different polarity combinations and squared in four different signal channels. Then the Vfour squared signals are added together with suitable polarities to 0btain the product SK in the output circuit .of the multiplier network, as will be illustrated by the following equations:

=s1+K2+A12+2sK+2KA1+2A1S =s2+K2+A12+2sK+2KAr2A1s increases the frequency spectrum of the waves K1 .and K2 to about 2000 cycles, although each of the delay networks have cutoff frequencies of the order of 660 cycles. Since high frequency componentslof the wave K12 are of low magnitude, the complex wave is passed through a high frequency booster network ||5 of any conventional type which equalizes the frequency components up to about `2000 cycles per second. The equalized complex wave derived from the high frequency booster network ||5 is applied to a phase distortion circuit H1 such, for example, as an RC lat tice network which has the property of delaying various frequency components by different amounts whereby the relative phases of the various harmonic components of the complex wave are changed to form a radically ldifferent wave shape. The phase distortion circuit I |1 also alters the square wave form of the complex wave form K12 so as to obliterate the sharp corners thereof` and to vary the lobes thereof over a wide variety of amplitudes. occurring in the distorted wave are removed by a limiting amplifier IIS from which is derived the nal extremely complex coding wave K. The complex coding Wave K is applied to the reciprocal circuit IUI and to th'e fixed contact T1 ofthe first transmit-receive switch' 50 of the circuit of Figure 1 as explained heretofore.

Judging from oscillographic observations of a complex coding wave of the type described, it is believed that the steps of multiplying and phase distorting the component waves K1 vand K2 effectively masks all significant 45.4 cycle and 50 cycle Sum output Excessively high lobes characteristics which ordinarily would be observable in the transmitted coded signal. 'I'herefore, the dual delay network wherein the separate code wave components are multiplied together and distorted, appears an economical and efiicient electrical device for generating a code Wave having a period as long as .4 second.

SIGNAL MULTIPLIER is described and claimed in a copending application of Aldal V. Bedford, U. S. Serial No. 517,967, filed January 12, 1944, and assigned to the same assignee as the instant application. The circuit utilizes the property of well known electrical de- SSK It will be understood that the term A1 in the above Equations is lthe D.C. `bias added to the A.C. waves to cause all of the signal amplitude variations to have the same polarity with respect to the squaring devices.

The squaring circuit illustrated employs a plurality of small copper oxide rectifiers known commercially as varistors Because of the particular variable resistance characteristics of the varistor, the current therethrough is substantially proportional to the square of the applied voltage over a reasonable applied voltage range in :a single polarity. The signal multiplier network is shown as including a first triode thermionic tube |33 having its grid electrode connected to the movable Contact of the first transmit-receive switch 50, whereby signals characteristic of either the coding wave K or the reciprocal thereof l/K may be applied to the tube grid cathode circuit. A second thermionic tube |35 has its grid electrode connected to the movable contact of the second transmit-receive Switch 51, whereby either the speech signals S or the blanked, received signals SK may be .applied to the tube grid-cathode circuit. The operation of the circuit will be explained hereinafter with the switches 50 and 51 in the transmitting position whereby the signals K and S, respectively, are applied to the grid-cathode circuits of the tubes I 33 and |35. Push-pull-output signals are derived from each of the tubes by means of connections yto the corresponding tube anode and cathode circuits as indicated in the drawings.

In order that the desired sum voltages be obtained, the signals S and K :are applied to a network of resistors in the following manner: Signals S and K respectively traverse resistors |31 and I 39 to provide a signal proportional to (S4-K) at point (S-l-K); the signals S and -K respectively traverse resistors I4! and |43 to provide signal (S-K); the signals S and -K respectively traverse resistors |45 and |41 to provide signal (-S-K) and the signals SK and K travl -r erse respectively resistors |49 and |5| `to provide signal (-S-I-K) Thus, at each of the four junction points, a sum of voltage is obtained as designated in the circuit diagram. As shown, the network also includes resistors |53 and |55 leading respectively from points (S-K) and (-S-}K) to ground, and resistors |51 and |59 leading respectively from points (S-l-K) and (-S-IO to the positive terminal of the source of bias voltage which is applied through a voltage divider ISI, An 8000-ohm resistance has been found sat- 13 isfaotory for the resistors |53, |55, |51, and |59 while 100,000 ohm resistance has been taken as the Value of resistors |31, |39, |4|, |03, |45, |41, |49, and |5l.

The sum voltages at the four points of the network are applied with bias voltage A and -A to four varistors |63, |65, |61 and |69 respectively, all of which control the `current through the common load resistor |1| to provide thereacross the product output voltage SK. The output voltage across resistor |1| is proportional to the sum of all the voltages which would have been generated if each varistor had Supplied current to a separate resistor, as indicated by the foregoing squaring equations. It is to be noted that the varistors |65 and |60 are connected with opposite polarities from the varistors |63 and |61, so that the D.C. bias voltage must be different. By reference respectively to the third and fourth equations it will be seen that the values (-S-l-K-A) and (S-K-A) are each preceded by another minus sign and included brackets before squaring to indicate properly mathematically the e'ect of the reversed connection on these two varistors. These five equations show that, ideally, only the desired voltage SK is produced across the output resistor |1|.

For compensating for small dissimilarities in the varistors and other circuit elements, it has been found desirable to provide variable resistors |13, and connected as voltage dividers in the anode circuits of the tubes |33 and |35 respectively for adjusting the relative amplitudes of -S and -K.

While in the foregoing the term multiplying circuit has been used to define the circuit, it will be seen that the circuit actually is a sort of modulator which is completely balanced in the sense that only the side band frequencies are produced, while the input frequencies and the harmonics thereof are suppressed.

The output signals SK derived from across the output resistor lil are applied to the movable contact of the third transmit-receive switch 6| whereby they may be selectively applied to either the reproducer |03 or to the first signal mixer 63 depending upon the desired operation of the circuit of Fig. 1.

\ SIGNAL RECIPROCAL CIRCUIT The reciprocal circuit |0| shown in Figures 1 and 8 of the drawings is described and claimed in the copending application of Carl A. Meneley, Serial Number 484,304, led April 23, 1943, and assigned to the same assignee as the instant application. In this circuit instantaneous reciprocal values of an applied coding wave K are obtained by means of an electrical network in which the wave K is clipped on both its positive cycle and on its negative cycle to produce a substantially rectangular wave, and in which the wave K and the rectangular wave are added together with one of them reversed in polarity preferably after the peaks of the positive and negative cycles of the wave K have been squashed or flattened somewhat. The circuit includes no appreciable capacitive or inductive reactances (the blocking capacitors in the circuit presenting negligible impedance) and, therefore, provides the reciprocal of substantially any applied signal wave form regardless of its frequency components.

Referring to Figure 8, the graph p represents a typical coding wave K which is applied to the input terminals |11 of the circuit. The graph s represents the reciprocal wave l/K, which is the sum of the iiattened wave K, represented by the graph q, of reversed polarity, and of the rectangular wave shown in graph r. The squashed or flattened wave q may be obtained by passing the wave p through a circuit that changes its resistance with a change in applied voltage. The rectangular wave T may be produced by clipping the positive and negative cycles of the wave q at the voltage levels t and u respectively, for example, close to the A.C. axis of the signal, and then by amplifying the clipped signal.

The wave K applied to the input terminals |11 may, if desired, be amplified by means of an amplifier tube |19 to provide a peak-to-peak amplitude, for example, of the order of volts. The amplified, K wave then is applied through a blocking capacitor |8| and a resistor |83 to a copper oxide rectifier unit which functions as a nonlinear resistor having the property of decreasing in resistance as the applied voltage increases. The resistor |83 is of high enough resistance so that the driving source for the non-linear resistance unit |85 is of high impedance whereby there is only a slight variation in the current iiow 25 .through the unit |85. The unit |85 may consist of a pair of copper oxide rectiiiers |81 and |89 connected to conduct current in opposite directions.

The voltage appearing across the non-linear unit |85 is the voltage wave q, which is the wave K having a attened wave form. This voltage is amplified .by a cathode biased vacuum tube |91, and appears across an anode resistor |93 and a portion of Athe anode resistor |95 of a second amplifier tube |91.

The rectangular wave 1 is produced, in this particular example, by applying the output of the tube |9| through a blocking capacitor |99 and a high impedance resistor 20| to a pair of diodes 203 and 205, which are connected to conduct in opposite directions. Resistors 201 and 209, of comparatively low resistance are connected in series with the diodes 203 and 205, respectively. A biasing voltage drop for opposing current flow through diode 205 is produced across the resistor 209 by connecting a source of voltage (not Shown) thereacross, a resistor 2| I being in series with the voltage source. 'I'he diodes 203 and 2F35 clip the applied wave q symmetrically about its A.C. axis,

50 because a voltage which causes current flow through the diode 203 and resistor 201 is built up across the capacitor |99 by the positive cycle pulses iiowing through the diode 205. Thus, the diodes 203 and 205 become conducting on alternate cycles when the signal voltage exceeds predetermined D.C. voltage values. The resulting rectangular wave r is amplified and reversed in polarity by the tube |91. The wave q and the flattened wave r add in the portion of the anode resistor |95 that is common to the tubes |0| and |01 to produce the desired reciprocal wave l/K shown in graph s.

If the wave q is liattened correctly, and if the waves q and T are added with the correct relative amplitudes, the resulting signal will be substantially a true reciprocal of the wave K. The only substantial departure from a true reciprocal si.,- nal will be where the wave K crosses the A.C. axis. Here the reciprocal value is innity whereas the maximum amplitude of the wave l/K necessarily has a finite limit. The waves q and r may be mixed with the correct relative amplitudes by adjusting a variable tap 2|3 on the anode resistor |95. The correct shaping of the flattened wave q may be obtained by selecting a non-linear resistor unit |85 having a suitable voltage-resistance characteristic and by adjusting the value of the variable resistor H83.

As previously noted, the above-described reciprocal circuit is purely resistive so that its operation is independent of frequency. The instantaneous voltage output of the circuit is always substantially the reciprocal of the instantaneous applied voltage. It follows that if the reciprocal circuit is adjusted to produce the reciprocal of an applied signal having one Wave form, the circuit will then always produce the reciprocal of an applied signal regardless of its wave forni. There are various ways of determining when the circuit has been adjusted to give substantially a true re ciprocaL One way is to connect the reciprocal circuit into the signalling system of Fig. l, and, while transmitting speech or music, adjust the 'ren sistor |83 and the variable tap 213 at the receiver until the speech or music has a minimum of distortion.

It should be understood that oppositely-connected diodes may be substituted for the copper oxide rectiers L87 and H89, described hereto-fore. When properly biased, the two diodes should be operated along the lower knee of their operating characteristic and in the proper region to shape the lwave K in the desired manner to provide the wave q.

It will be understood that the reciprocal circuit is not limited to the particular circuit components illustrated since the waves q and r may be derived from the wave K in various other ways, and since the two waves may be combined by means of a variety of other circuits.

MoroR-DRrvEN SELECTOR SWITCH MEcHANIsM Referring to Figures 9 and l0, portions o1" a motor-driven selector switch mechanism are shown, which in any known manner may change the characteristics of the complex coding wave K slightly at intervals of .4 second whereby the coding wave characteristics are continuously changed over a period of approximately one hour and six minutes for a predetermined initial setting of a plurality of differential-speed coding Contact discs of the type shown in Fig. 9. A complete mechanism for providing differential rotation of said disks is described in said copending application, but it may be varied in any known manner.

Predetermined rotational arrangements of the coding discs with respect to their complementary fixed brush holders permits continuous variation of the complex coding wave characteristics for a period of the order of one hour and six minutes at a motor 'speed of 1GO R. P. M. By means of detent mechanisms on each of the switch contact discs, the relative disc rotational arrangement may be changed manually at hourly intervals to provide an entirely different coding wave for each hour of operation, whereby the coding wave characteristics may be made substantially non-repetitive over an extremely long period.

A switch motor drive at the transmitter unit may be synchronized with a similar switch motor drive at the receiver unit in any manner known in the art such, for example, as the inclusion of a synchronizing tone in the transmitted signal. Alternative arrangements may employ crystal, or fork, controlled generators at both transmitter and receiver which operate at substantially constant frequencies to synchronize the motors over extended operating periods. Various well known methods or" phasing synchronizing motors may be employed if desired.

More detailed construction of one of the con'- tact discs is shown in Figures 9 and 10, wherein a typical one of the contactor discs 2&5 is mounted upon a tubular shaft 239 and adjustably keyed thereto by a diametrically disposed two section telescoping pin 2%3. The pin normally is expanded by an intermediate compression spring 265 to project at opposite sides of the shaft and engage a pair of notches 2S? provided in the inner face of the disc 2135. In effect, the pin 2&3 and the notches 2537 which extend entirely around the inner face oi the disc comprise a detent mechanism which permits the disc to be rotated manually for presetting the individual disc position with respect to a motor driven gear mechanism.

Concentrically arranged around the central axis of the disc are three series of side face contactors 269, 2l! and 2'l'3, preferably comprising silver segments insulated from each other. The silver segments are of three types which are 1ocated in irregular order of different radial dimensions, but all of which have a portion in the same intermediate radial circular path. Thus the short intermediate contactors 2li! denne neutral positions of the switches; the long outer contactors 273 deiine positive posit-ions of the switches and long inner contactors 26% provide negative positions or" the switches in terms of the pulse components of the coding wave. As shown, the inner edges of the long outer contactors 213 align oircumferentially with the inneredges of the short intermediate contactors 2li but the radial length of such long outer contactors 273 is approximately twice that of the intermediate contactors 2li. The outer edges of the long inner contactors 269 align circumferentially with the outer edges of the short intermediate contactors 2li but the radial length of such long inner contactors 259 is also approximately twice that of the intermediate contactors 2li.

Hence, in the rotation of the disc, the intermediate contactors 27! are arranged to wipe successively one intermediate series of brushes Zl; the long outer contactors 2!3 successively wipe said intermediate series of brushes 275 and an outer series` of brushes 2TH in pairs, one brush of each series being wiped simultaneously. Similarly, the long inner contactors 269 successively wipe said intermediate series of brushes N5 and an inner series of brushes 2?! in pairs, one brush from each of the latter series being wiped simultaneously.

Referring to Figure l0, each of the brush series 215, 2l?, ZTS has live brushes, corresponding brushes of each series being spaced at similar or slightly diierent angular intervals so as to be wiped concurrently by the contactors on the contactor disc 2&35. The sets of brushes are mounted upon an insulating plate 281, which in assembled condition, is in close proximity to the contacter side of the rotatable disc M5, in order that the respective contactors may contact with and bridge any pair of brushes to be selected, so as to close predetermined circuits for the generation of the l coding waves.

Thus the invention disclosed comprises an improved method of and means for generating and synchronizing extremely complex electrical waves which are generated by selecting predetermined delayed signal components from a plurality of dierently-pulse-excited delay networks, and wherein said derived signal components are combined to form said complex wave. The invention is illustrated herein, by way of example, as a component of a secret telecommunication system wherein such complex wave generators are employed to provide coding and decoding waves at a transmitter and receiver, respectively. The invention comprises a synchronizing system for such coding wave generators wherein the synchronizing signals are transmitted from transmitter to receiver to synchronize one generator in each unit, and novel local synchronization is provided for the second generator in each unit. A motor driven switching mechanism is briefly described which provides continuously changing selection of the delayed signal components in each of the delay networks in a manner whereby the derived complex signal is non-repetitive for an extended time interval.

I claim as my invention:

i. A device including a plurality of wave generators, a delay network excited by waves from one of said generators, means for deriving from said network different successively delayed components of said exciting waves, predetermined ones of said derived delayed wave components being successively reversed in polarity, means for combining said derived wave components of both polarities to generate a wave which is a predetermined harmonic of said exciting wave frequency, and means for synchronizing at least one other of said generators at a predetermined subharmonic of said generated harmonic wave.

2. In a system comprising a plurality of devices of the type described in claim 1, means for synchronizing a corresponding one of said exciting wave generators of each of said devices whereby at least one other generator of each of said devices is synchronized with respect to a corresponding other one of said generators of another of said devices.

3. A device including a plurality of pulse generators, a delay network excited by pulses from one of said generators, means for deriving from said network different successively delayed pulse components of said exciting pulses, means for phase inverting predetermined ones of said delayed pulse components, means for combining predetermined ones of said derived and said phase-inverted pulse components to generate a wave having a predetermined harmonic relation to the frequency of said exciting pulses, and means for applying said generated wave to synchronizeanother of said generators at a predetermined sub-harmonic of said generated wave frequency.

4. In a system comprising a plurality of devices of the type described in claim 3, means for synchronizing a corresponding one of said pulse exciting wave generators of each of said devices whereby at least one other of said generators of each of said several devices is synchronized with respect to a corresponding other one of said generators of another of said devices.

5. A device including a plurality of pulse generators, a delay network excited by pulses from one of said generators, means for deriving from said network different successively delay pulse components of said exciting pulses, amplier means for phase inverting predetermined ones of said delayed pulse components, means for combining predetermined ones of said derived and said phase-inverted pulse components to generate a wave having a predetermined harmonic relation to the frequency of said exciting pulses, and means for applying said generated wave to synchronize another of said generators at a predetermined sub-harmonic of said generated wave ireticencia 6. Apparatus of the type described in claim 3 including means for selecting predetermined signal components from each of said generators, and .means for combining said selected signal components to generate a complex electrical wave.

'7. In a system comprising a plurality of devices of the type described in claim 3, means for synchronizing a corresponding one of said pulseexciting wave generators of each of said devices whereby others of said generators of said several devices are synchronized with respect to corresponding generators of another of said devices, means for selecting predetermined signal components from said generators of each of said devices, and means forming parts of each of Said devices for combining said selected signal components therein to generate a plurality of synchronized complex electrical waves.

8. A synchronizing system for a plurality of wave generators comprising means for synchronizing .two of said generators, separate delay networks excited by each of said synchronized generators, means for deriving from each of said networks different successively delayed signal components of each of said exciting generators, means for combining said derived delayed signal components from each of said networks to generate separate waves each having predetermined harmonic relation to the frequency of the corresponding one of said exciting generators, and means for applying each of said generated harmonic waves to synchronize separately with each of said exciting generators others of said generators at predetermined sub-harmonics of said harmonic Waves.

9. A device including a plurality of wave generators, a delay network excited by waves from one of said generators, means for driving from said network different successively delayed components of said exciting waves, means for combining said derived wave components to generate a wave which is a. predetermined harmonic of said exciting wave frequency, and means for synchronizing at least one other of said generators at a predetermined sub-harmonic of said generated harmonic wave.

10. In a system utilizing a plurality of electrical waves, the method comprising delaying oneV of said waves, deriving from said delayed wave different successively delayed wave components, combining said derived wave components to generate another wave which bears a predetermined harmonic relation to said delayed Wave, and synchronizing another of said waves at a predetermined subharmonic of said harmonic wave.

11. In a system utilizing a plurality of electrical waves, the method comprising delaying one of said waves, deriving from said delayed wave diierent successively delayed wave components, successively reversing in polarity predetermined ones of said derived delayed wave components, combining said derived wave components of both polarities to generate a wave which bears a predetermined harmonic relation to said delayed wave, and synchronizing another of said waves at a predetermined subharmonic of said generated harmonic wave.

12. The method described in claim 11 including the step of synchronizing a third of said electrical waves with said delayed wave, and synchronizing a fourth of said waves at a predetermined subharmonic of a harmonic of said third Wave.

KARL R. WENDT. 

